The quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive.
To improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is reformed with water to produce carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas intermediate is converted to higher hydrocarbon products by processes such as the Fischer-Tropsch Synthesis. For example, fuels with boiling points in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes may be produced from the synthesis gas.
Current industrial use of methane as a chemical feedstock proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming, which is the most widespread process, or by dry reforming or by autothermal reforming. Steam reforming currently is the major process used commercially for the conversion of methane to synthesis gas, proceeding according to Equation 1.
Although steam reforming has been practiced for over five decades, efforts to improve the energy efficiency and reduce the capital investment required for this technology continue. For many industrial applications, the 3:1 ratio of H2:CO products is problematic, and the typically large steam reforming plants are not practical to set up at remote sites of natural gas formations.
Methane residence times in steam reforming are on the order of 0.5-1 second, whereas for heterogeneously catalyzed partial oxidation, the residence time is on the order of a few milliseconds. For the same production capacity, syngas facilities for the partial oxidation of methane can be far smaller, and less expensive, than facilities based on steam reforming. A recent report (M. Fichtner et al. Ind. Eng. Chem. Res. (2001) 40:3475–3483) states that for efficient syngas production, the use of elevated operation pressures of about 2.5 MPa is required. Those authors describe a partial oxidation process in which the exothermic complete oxidation of methane is coupled with the subsequent endothermic reforming reactions (water and CO2 decomposition). This type of process can also be referred to as autothermal reforming or ATR, especially when steam is co-fed with the methane. Certain microstructured rhodium honeycomb catalysts are employed which have the advantage of a smaller pressure drop than beds or porous solids (foams) and which resist the reaction heat of the total oxidation reaction taking place at the catalyst inlet. The honeycomb is made by welding together a stack of rhodium foils that have been microstructured by means of wire erosion or cutting.
The catalytic partial oxidation (“CPOX”) or direct partial oxidation of hydrocarbons (e.g. natural gas or methane) to syngas has also been described in the literature. In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The partial oxidation of methane yields a syngas mixture with a H2:CO ratio of 2:1, as shown in Equation 2.
This ratio is more useful than the H2:CO ratio from steam reforming for the downstream conversion of the syngas to chemicals such as methanol or to fuels. Furthermore, oxidation reactions are typically much faster than reforming reactions. This allows the use of much smaller reactors for catalytic partial oxidation processes than is possible in a conventional steam reforming process.
While its use is currently limited as an industrial process, the exothermic direct partial oxidation or CPOX of methane has recently attracted much attention due to its inherent advantages, such as the fact that due to the significant heat that is released during the process, there is no requirement for the continuous input of heat in order to maintain the reaction, in contrast to steam reforming processes.
The current interest in partial oxidation processes has resulted in various improvements in the technologies associated with that process, including catalyst composition, catalyst structure, reactor structure, and operating parameters. One aspect that has not received as much attention is the technology associated with the injection of feed gases into the partial oxidation reactor. In particular, it is necessary to feed methane and an oxygen-containing gas into the reactor under conditions of elevated temperature and pressure. The same feed conditions that are conducive to efficient operation of the partial oxidation process, however, are conducive to reactions that are less desirable, and possibly even hazardous, such as the ignition and combustion of the feedstock. At the same time, it is desirable to mix the feed gases as completely as possible, to as to maximize the efficiency of the catalytic reaction. It is particularly desirable to avoid pre-reaction and pre-ignition of the gases.
As described in U.S. Pat. No. 6,267,912, which is incorporated herein by reference, catalytic partial oxidation processes attempt to eliminate gas phase oxidation reactions entirely, so that all of the partial oxidation reactions take place on the catalyst surface. The reactants are contacted with the catalyst at a very high space velocity, so that gas phase reactions are minimized. Gas phase reactions are undesirable because they can increase the occurrence of undesired combustion reactions (producing steam and carbon dioxide), damage the catalyst, and accelerate its deactivation.
Hence, there is a need for a method and apparatus for mixing partial oxidation feed gases that achieves thorough mixing, and if necessary preheat, and provides an even distribution of the mixed gases across the catalyst surface, while minimizing or preventing undesired gas phase reactions.